The use of a Hartmann-Shack wavefront apparatus to determine ocular aberrations of subjects by measuring a wavefront emerging from an eye is known. To produce such an emerging wavefront, a laser beam is injected into the eye, and a spot of light formed on the retina is reflected from the retina to produce a wavefront emerging from the eye. The emerging wavefront is distorted by the eye's aberrations as it passes through the cornea, lens and other portions of the eye. Upon exiting the eye the wavefront is directed onto a sensor and the output of the sensor is provided to a calculator of aberrations.
A relatively small spot on the retina is desirable for producing a retinal reflection because it operates as a point source when producing an emerging wavefront. However, when a beam of light from a laser is injected into the eye, the size of the spot on the retina is affected by the refractive power of the cornea and the lens, as well as aberrations of the eye.
To achieve a suitably small spot on the retina, some conventional wavefront apparatus actively or statically compensate for myopia or hyperopia, and other aberrations. For example, in one apparatus, active compensation is achieved by movement of a focus corrector in a trombone stage that is located in front of subjects' eyes to adjust the location of the laser beam focus relative to each subject's retina.
In another conventional apparatus, a focusing lens is selected to have a long effective focal length to focus light onto subjects' corneas. In such static apparatus, a focusing lens, located at a fixed location, is arranged to project a geometrical focus onto subjects' corneas such that the impact of hyperopia and myopia is reduced and the impact of the aberrations of the subjects' eyes is also reduced. Since, the beam diverges after passing through a cornea (i.e., after passing through geometrical focus) the spot size at the retina is often larger than desired.